The present invention relates broadly to radar systems which employ constant false alarm rate (CFAR) receivers based on phase discrimination logic and more particularly, to a CFAR decoder for use therein having improved sensitivity in the detection of desired echos in the presence of undesired echo interference or receiver noise.
Phase discrimination CFAR's are widely employed in the radar field to prevent a significant increase in false alarm rate (FAR) when interference, such as rain, chaff, active jamming or pulses from other radars, for example, are encountered. For more detail description of the principles of CFAR, reference is made to section 5.8 of the Radar Handbook which was edited by M. I. Skolnick (McGraw-Hill, 1970). For the purposes of this application, it will only be necessary to consider those CFAR processes which discriminate between desired and undesired echos solely on the basis of the phase patterns of the signal content that is received in relation to that which is transmitted. The objective of the phase discrimination type CFAR is to control the false alarm rate in noise or any random-phase interference, independent of both the mean amplitude of the interference and its amplitude distribution. Desired echos are detected in the presence of undesired interference, for the most part, by how well the phase code of the desired echos correlates with the phase code of the signal transmitted.
Phase discrimination CFAR is often called "Dicke-fix CFAR" when the radar transmits a pulse having essentially constant phase throughout and "CPACS (Coded Pulse Anti-Clutter System)" when a series of contiguous sub-pulses are transmitted, having a desirable phase pattern. The receiver implementation may be the same in either case; however, because the performance of CPACS in many types of interference is superior to Dicke-fix CFAR, it is more widely used. Where the term CPACS is employed in the following discussion, it should be understood that Dicke-fix CFAR is also implied.
In most modern radars, the CPACS is generally preceded by a linear, wide-dynamic range doppler filter or MTI, usually of the digitally implemented variety, which operates on the received radar echo signal and generates two signals, I and Q, which are representative of the in-phase and quadrature components of the echo vector, normally denoted by A cos .phi. and A sin .phi., respectively, where A is the amplitude of the signal and .phi. is the phase relative to a conventional local reference oscillator. Existing digital CPACS designed for MTI or doppler-type radar processors generally utilize only one of the two echo vector components, I and Q. Exemplary of this type of CPACS is the one disclosed in U.S. Pat. No. 3,887,918, issued to John S. Bailey et al on June 3, 1975. The sacrifice in sensitivity encountered in the operation of these one-bit CPACS decoders was tolerated in the early days of radar signal processing in order to maintain the cost of the MTI system to a reasonable level. However, more recently CPAC systems have been developed which make use of both the I and Q vector components of the echo signals, primarily due to the reduction in cost and size of the electronic components used in the design thereof. In these CPAC systems, the polarities of the I and Q vector components are used to measure the phase pattern of the coded echo signal in 90.degree. segments, thus potentially providing a higher degree of sensitivity over that of the one-bit decoders in discriminating the desired echo signals from interference. CPACS decoders of this type may be described as quadraphase or 2-bit designs.
While it is recognized that the 2-bit CPACS designs significantly improve the sensitivity of the radar system, it is further believed that the full benefits-cost ratio (dB/$) has not yet reached an optimum with respect to additional discrimination inherent in the phase pattern of the coded echo signals. For example, it can be shown that a sensitivity improvement of 0.6 dB can be achieved by increasing the phase quantization from a 2-bit design to a 3-bit design. It is therefore applicant's intention to disclose herebelow such a 3-bit CFAR system having an improvement in sensitivity and a favorable benefit-cost ratio.